OP I am sorry for your loss, much as though you think this situation is unfair, disrespectful to your dad etc The money was your mums to do as she wished with, she made a new will and did just that, unfair as it may well feel to you and your other sibling.
As to your brother, did he put influence on her to change her will in his favour? who knows, its possible, but if that was the case then he would surely have made sure he got everything. He and his partner were caring for your mum, dont underestimate just how hard that is, would you feel you were still entitled to your third if you mum had lived 1, 3, 5 or 10 more years with your brother and partner doing all her daily care?
You said you spoke to her frequently, did you have any concerns about her mental well being, her capacity? you may not have been seeing her but if you were talking to her regularly you would and do notice difference. I live abroad speak to my mum most days, I KNOW when she is ill or things are not good, she doesn't have to say it I can hear the difference in her voice.
Finally I had a very nasty situation regarding money a house and an ex, I was so so pissed off angry, bitter and hated him with a passion but all those emotions only hurt me, made me ill, I was becoming someone I didnt like, it was consuming me and my life, in the end I settled, cost me tens of thousands but it was like a weight was lifted. Holding on to anger, bitterness and hate is like holding a hot coal it only hurts you
I wish you luck, but more importantly peace and closure